Comm Sys Lab

8/3/2019 Comm Sys Lab

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1. Modulation: An Introduction

One way to communicate a message signal whose frequency spectrum does not fallwithin that fixed frequency range, or one that is otherwise unsuitable for the channel, is to

change a transmittable signal according to the information in the message signal. This

alteration is called modulation, and it is the modulated signal that is transmitted. Thereceiver then recovers the original signal through a process called demodulation.

Modulation is a process by which a carrier signal is altered according to information in amessage signal. The carrier frequency, denoted Fc, is the frequency of the carrier signal.

The sampling rate, Fs, is the rate at which the message signal is sampled during the

simulation.

The frequency of the carrier signal is usually much greater than the highest frequency of

the input message signal. The Nyquist sampling theorem requires that the simulationsampling rate Fs be greater than two times the sum of the carrier frequency and the

highest frequency of the modulated signal, in order for the demodulator to recover the

message correctly.

1.1 Baseband Versus Passband Simulation

For a given modulation technique, two ways to simulate modulation techniques are calledbaseband and passband. Baseband simulation requires less computation. The

MATLAB Communication toolbox supports baseband simulation for digitalmodulation and passband simulation for analog modulation. In this tutorial, baseband

simulation will be used.

1.2 Digital Modulation Techniques

1.2.1 Amplitude Shift Key (ASK) Modulation

In this method the amplitude of the carrier assumes one of the two amplitudes dependenton the logic states of the input bit stream. A typical output waveform of an ASK

modulator is shown in Fig. 1.

Digital Communication Systems Lab.

Name:

Group

Simulation using Matlab


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Fig. 1: ASK Modulation

1.2.2 Frequency Shift Key (FSK) Modulation

In this method the frequency of the carrier is changed to two different frequenciesdepending on the logic state of the input bit stream. The typical output waveform of an

FSK is shown in Fig. 2. Notice that a logic high causes the centre frequency to increase to

a maximum and a logic low causes the centre frequency to decrease to a minimum.


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Fig. 2: FSK Modulation

1.2.3 Phase Shift Key (PSK) Modulation

With this method the phase of the carrier changes between different phases determined

by the logic states of the input bit stream.

There are several different types ofPhase Shift Key (PSK) modulators. These are:

Two-phase (2 PSK)

Four-phase (4 PSK)

Eight-phase (8 PSK)

Sixteen-phase (16 PSK)

Two-Phase Shift Key ModulationIn this modulator the carrier assumes one of two phases. A logic 1 produces no phase

change and a logic 0 produces a 180 phase change. The output waveform for thismodulator is shown in Fig. 3.


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Fig. 3: 2PSK or BPSK Modulation

Four-Phase Shift Key Modulation

With 4 PSK or QPSK, 2 bits are processed to produce a single-phase change. In this case

each symbol consists of 2 bits. The actual phases that are produced by a 4 PSK modulator

are shown in Table 1.

Table 1: Bits and Phases for 4 PSK modulation

Bits Phase

00 45

01 135

10 315

11 225

From Table 1, s signal space diagram or signal constellation can be drawn as shown in

Fig. 4. Note from Fig. 4 that from any two closest bits sequences, there is only one bit

change. This is called Gray Coded scheme. For example, bit sequence 00 has one bit

change for its closest bit sequences 01 and 10.


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Fig. 4: 4PSK constellation

Eight-Phase Shift Key Modulation

With this modulator 3 bits are processed to produce a single-phase change. This means

that each symbol consists of 3 bits. Fig. 5 shows the constellation and mapping of the 3-bit sequences onto appropriate phase angles.

Fig. 5: 8 PSK signal constellation

Higher Order PSK modulation schemes

Modulation schemes like 16 PSK, 32 PSK and higher orders can also be designed and

represented on a signal space diagram.

/2

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0

0001

11 10

/2

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0

000010

111

100

001011

110

101

45


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1.2.4 Quadrature Amplitude Modulation (QAM)

QAM, is a method for sending two separate (and uniquely different) channels of

information. The carrier is shifted to create two carriers namely the sine and cosineversions. The outputs of both modulators are algebraically summed and the result of

which is a single signal to be transmitted, containing the In-phase (I) and Quadrature (Q)information. The set of possible combinations of amplitudes, as shown on an x-y plot, isa pattern of dots known as a QAM constellation.

Consider the 16 QAM modulation scheme. With this modulator, 4 bits are processed toproduce a single vector. The resultant constellation consists of four different amplitudes

distributed in 12 different phases as shown in Fig. 6.

Fig. 6: 16 QAM Constellation


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2. QAM Modulation and MATLAB

To modulate a signal using digital modulation with an alphabet having M symbols, start

with a real message signal whose values are integers between 0 and M. Represent the

signal by listing its values in a vector, x. Alternatively, a matrix can be used to represent a

multichannel signal, where each column of the matrix represents one channel.

For example, if the modulation uses an alphabet with 8 symbols, then the vector [1 2 3 10 4 4 2 5]' is a valid single-channel input to the modulator. As a multichannel example,

the two-column matrix

[2 3;

3 3;

7 3;0 3;]

defines a two-channel signal in which the second channel has a constant value of 3.

Problem definition: A simulation study must be carried out for a binary data stream

that has to be transmitted over a channel known as Additive White Gaussian Noise

(AWGN) Channel using 16QAM modulation scheme.

Solution: Use MATLABCommunication Toolbox to simulate the system. The latter

will consist of a 16QAM baseband modulator, AWGN channel, and 16QAMdemodulator. The system's bit error rate (BER) is computed and also the transmitted and

received signals will be displayed in a scatter plot.

The table below indicates the key tasks in solving the problem, along with relevantfunctions from the MATLAB

Communications Toolbox. The solution arbitrarily

chooses baseband 16QAM as the modulation scheme and AWGN (additive white

Gaussian noise) as the channel model.

Task Function

Generate a random binary data stream randint

Modulate using 16-QAM

Add white Gaussian noise awgn

Create a scatter plot scatterplot

Demodulate using 16-QAM

Compute the system's BER biterr

QAMMOD

QAMDEMOD


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The sections below describe each step in more detail, introducing M-code along the way.

To view all the code in one editor window, enter the codes in the MATLAB CommandWindow.

edit commdoc_mod

2.1 Generate a Random Binary Data Stream

Use the randint function to create a column vector that lists the successive values of abinary data stream. Set the length of the binary data stream to 30,000.

The code below creates a stem plot of a portion of the data stream, showing the binaryvalues. Figure 1 shows the stem plot of the input data stream.

%% Setup

% Define parameters.

M = 16; % Size of signal constellationk = log2(M); % Number of bits per symbol

n = 3e4; % Number of bits to process = 30,000

%% Signal Source

% Create a binary data stream as a column vector.

x = randint(n,1); % Random binary data stream

% Plot first 40 bits in a stem plot.

stem(x(1:40),'filled');

title('Random Bits');xlabel('Bit Index'); ylabel('Binary Value');


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2.2 Prepare to Modulate

Each 4-tuple of values from x is arranged across a row of a matrix, using the reshape

function in MATLAB, and then the bi2de function is applied to convert each 4-tuple to acorresponding integer. (The .' characters after the reshape command form the

unconjugated array transpose operator in MATLAB. Figure 2 shows the random symbols

being generated.

%% Bit-to-Symbol Mapping

% Convert the bits in x into k-bit symbols.

xsym = bi2de(reshape(x,k,length(x)/k).','left-msb');

%% Stem Plot of Symbols

% Plot first 10 symbols in a stem plot.

figure; % Create new figure window.

stem(xsym(1:10));

title('Random Symbols');

xlabel('Symbol Index'); ylabel('Integer Value');


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2.3 Modulate Using 16-QAM

The dmodce function implements a 16-QAM modulator. xsym from above is a column

vector containing integers between 0 and 15. The qammod function can now be used to

modulatexsym using the baseband representation. Note thatMis 16, the alphabet size.

%% Modulation

% Modulate using 16-QAM.

y = modulate (modem.qammod(M),x) ;

The result is a complex column vector whose values are in the 16-point QAM signalconstellation. A later step in this example will show what the constellation looks like.

2.4 Add White Gaussian Noise (AWGN) Channel

Applying the awgn function to the modulated signal adds white Gaussian noise to it. Theratio of bit energy to noise power spectral density, Eb/N0, is arbitrarily set at 10 dB. The

expression to convert this value to the corresponding signal-to-noise ratio (SNR) involves

k, the number of bits per symbol (which is 4 for 16-QAM).The factor kis used to convert Eb/N0 to an equivalent Es/N0 , which is the ratio of

symbol energy to noise power spectral density.


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%% Transmitted Signal

ytx = y;

%% Channel

% Send signal over an AWGN channel.

EbNo = 10; % In dB

snr = EbNo + 10*log10(k);

ynoisy = awgn(ytx,snr,'measured');

%% Received Signal

yrx = ynoisy;

2.5 Create a Scatter Plot

The scatterplot function is applied to the transmitted and received signals. This shows

how the signal constellation looks like and how the noise distorts the signal. In the plot,

the horizontal axis is the In-phase (I) component of the signal and the vertical axis is the

Quadrature (Q) component. The code below also uses the title, legend, and axis functionsin MATLAB to customize the plot. Figure 3 shows the received signal being distorted.

%% Scatter Plot

% Create scatter plot of noisy signal and transmitted

% signal on the same axes.

h = scatterplot(yrx(1:5e3),1,0,'g.');

hold on;

scatterplot(ytx(1:5e3),1,0,'k*',h);

title('Received Signal');

legend('Received Signal','Signal Constellation');

axis([-5 5 -5 5]); % Set axis ranges.

hold off;


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2.6 Demodulate Using 16-QAM

Demodulation of the received 16-QAM signal is done by using the ddemodce function.The result is a column vector containing integers between 0 and 15.

%% Demodulation

% Demodulate signal using 16-QAM.

2.7 Convert the Integer-Valued Signal to a Binary Signal

The previous step produced zsym, a vector of integers. To obtain an equivalent binarysignal, use the de2bi function to convert each integer to a corresponding binary 4-tuple

along a row of a matrix. Then use the reshape function to arrange all the bits in a single

column vector rather than a four-column matrix.

%% Symbol-to-Bit Mapping

% Undo the bit-to-symbol mapping performed earlier.

z = de2bi(zsym,'left-msb'); % Convert integers to bits.

% Convert z from a matrix to a vector.

z = reshape(z.',prod(size(z)),1);

zsym = modulate (modem.qamdemod(M),yrx) ;

2.8 Compute the System's BER

The biterrfunction is now applied to the original binary vector and to the binary vector

from the demodulation step above. This yields the number of bit errors and the bit errorrate.

%% BER Computation

% Compare x and z to obtain the number of errors and

% the bit error rate.

[number_of_errors,bit_error_rate] = biterr(x,z)

The statistics appear in the MATLAB Command Window. Results might vary because

the example uses random numbers.

number_of_errors = 71

bit_error_rate = 0.0024


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2.9 Simulating different Constellations

1. Choose different values for M and repeat the above procedure for different Eb/No values

2. Choose different constellation types and repeat the above procedure for different Eb/No values

3. Fill in the table below with the BER values obtained for each Modulation technique

4. Plot the results you've written in the figure below

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